Recently, it has been reported that particular fluorescein derivatives that per se have almost no fluorescent property react with nitrogen monoxide under a neutral condition to provide a triazole compound having a strong fluorescence intensity, and the triazole derivative can emit intense fluorescence at wavelength of around 515 nm under excitation light at a long wavelength of around 495 nm (U.S. Pat. No. 5,874,590). When these fluorescein derivatives are used as an agent for nitrogen monoxide measurement, the excitation light can be easily cut off with a fluorescence filter provided on an ordinary fluorescence microscope, and intracellular nitrogen monoxide concentration can be conveniently measured by measuring fluorescence in individual cells.
Diaminorhodamine derivatives that can efficiently react with nitrogen monoxide under a neutral condition, and give a triazole derivative having superior fluorescence intensity have been proposed (U.S. Pat. No. 6,201,134). The fluorescence of the diaminorhodamine derivatives shifts to a longer wavelength side compared with the aforementioned fluorescein derivatives, thus the fluorescence does not substantially overlaps the auto fluorescence of cells, and the fluorescence does not attenuate even in an acidic region. Therefore, the fluorescence of the diaminorhodamine derivatives does not damage living tissues and cells, and enable measurement of nitrogen monoxide in a fluorescent region of a longer wavelength than in the auto fluorescent region of cells.
Indacene derivatives, which are known to have an ion capturing moiety useful for measurement of alkali metal ions or cations (Japanese Patent Laid-Open Publication (KOKAI) Nos. 10-338695 and 11-5796). However, no attempt has been reported for measurement of nitrogen monoxide by using the fluorescence chromophore of the indacene derivatives.